The present invention is directed to a phase-change ink composition. More particularly, the present invention is directed to an ink that is a solid at room temperature, a single-phase liquid at a jetting or application temperature and that, upon solidifying, forms an elastic phase and a crystalline phase.
Ink jet inks are well-known in the art and remain a developing technology. These inks are used in a wide variety of applications, varying from home printing application to large scale, high-speed commercial printing operations.
There are two principal types of ink jet inks, namely, liquid inks and solid inks. Current liquid ink technologies focus on aqueous dispersions of various constituents that typically include, at a minimum, a carrier, a colorant or pigment, and one or more polymer-based constituents for providing integrity to the print media These polymer based constituents are generally used to impart desired physical and chemical properties for the final use of the ink. Examples of these polymer-based constituents include binders, thickeners, thixotropic agents, coating aids and the like.
One type of solid ink used in ink jet printing is a hot-melt ink. These inks are typically non-aqueous. In a hot-melt ink or thermal ink process, the ink is melted by a heater in the printing head or device and thus is a liquid at its application or operating temperature. Typically, the vehicle for carrying the dye or colorant has a low critical temperature to facilitate melting and thus use of the solid ink. The ink is heated and melted and subsequently jetted as a xe2x80x9cdropletxe2x80x9d from the printing apparatus. Upon contact with the printed media the molten ink rapidly solidifies. The ink remains on the surface of the media because of this rapid solidification This provides increased print quality in the form of higher print density and smaller, more regular dot size. It will be recognized that there are many advantages to hot-melt inks. One significant advantage is the decreased potential to spill the ink during handling.
Early hot melt inks were problematic vis-à-vis temperature stability. That is, the dyes in the inks were susceptible to thermal degradation. As such, pigments were adopted for use in favor of dyes. However, problems arose with respect to settling of the pigments or concentration of the pigments at ink-melt boundaries.
Later developments saw the rise of hot-melt vehicles formed of amide compounds. Mixtures of tetra-amide and mono-amide compounds were used, which tetra-amide compounds were formed by reacting ethylene diamene, dimer acid and stearic acid. The mono-amides used were stearamide. Tackifiers were added to promote adhesion to the underlying substrate or media. Anti-oxidants and plasticisers were also known to be used to increase flexibility and lower the melt viscosity.
Further developments saw the use of manipulated polymer cross-linking to achieve the needed phase change without using acidic polymers. It was found that acidic polymers resulted in increased corrosion of the printing apparatus. In one known hot melt ink, a reversible cross-linking material, namely oxyaluminum octotate is mixed with a saturated, long-chain linear alcohol, a pigment, and anti-oxidant, and a corrosion inhibitor.
It has, however, been found that the known hot-melt inks require relative high operating temperatures. That is, the application temperature required for the printing device (i.e., melting the solid ink) is at least about 150xc2x0 C. In addition, the print formed by currently known hot melt inks do not provide sufficient mechanical strength. That is, the integrity of the printed media may not necessarily meet rigorous standards and specifications for use in commercial applications.
Accordingly, there exists a need for an ink that is a single phase liquid at operating temperature and solidifies upon printing. Desirably, such an ink, upon solidifying, forms two phases, namely, an elastic phase and a crystalline phase. The elastic phase is reinforced by the crystalline phase to provide durable end user properties with high mechanical strength.
Such an ink provides high integrity text or printed media Most desirably, such a hot melt ink can be used at temperatures less than about 150xc2x0 C., and preferably, such inks can be used at jetting or operating temperatures of about 110xc2x0 C. to about 130xc2x0 C.
A hot-melt ink for use with an ink jet printing apparatus is a liquid at about 100xc2x0 C. to about 130xc2x0 C. and solidifies to a two-phase solid having an elastic phase and a crystalline phase. The ink has a formulation including a carrier, a first plasticizer, a linear block copolymer, a block copolymer plasticizer, a flow additive, and a colorant.
The block copolymer plasticizer, in combination with the linear block copolymer forms an elastic phase of the ink upon solidifying. The first plasticizer exhibits sufficiently low viscosity at elevated temperatures to permit ejection of the liquid ink from the printing apparatus at a desired operating temperature.
The present ink formulation provides numerous benefits over known solid hot-melt ink jet ink formulations. Principally, however, it has been found that the present hot-melt ink functions at operating temperatures of about 115xc2x0 C., and thus provides significant advantages over known hot-melt ink formulations that require heating to temperatures of at least about 150xc2x0 C.
In a preferred hot-melt ink formulation in accordance with the present invention the carrier is a fatty acid, preferably stearic acid. Preferably, the carrier is present in a concentration of about 45 percent to about 95 percent of the ink, and most preferably in a concentration of about 78 percent of the ink. Alternately, the carrier can be palmitic acid, myristic acid or the like.
In the preferred formulation, the first plasticizer is an aromatic hydrocarbon resin. Preferably, the first plasticizer is present in a concentration of about 0.1 percent to about 25 percent of the ink, and most preferably about 15 percent of the ink.
The preferred hot-melt ink includes a flow additive, preferably constituting a fluorinated polyolefin copolymer in a concentration of about 0.5 percent to about 10 percent of the ink, and more preferably about 1.0 percent of the ink.
Preferably, the block copolymer plasticizer is a polyvinyl acetal, and most preferably polyvinyl butyral in a concentration of about 0.5 percent to about 10 percent of the ink and more preferably in a concentration of about 1.0 percent of the ink.
In a most preferred ink formulation the linear block copolymer is a tri-block (A-B-A) copolymer. A preferred A-B-A copolymer is a styrene-butadiene-styrene block copolymer in a concentration of about 0.5 percent to about 5.0 percent of the ink, and more preferably in a concentration of about 1.0 percent of the ink. Alternately, the A-B-A copolymer can be a styrene-isoprene-styrene block copolymer.
In a preferred ink formulation, the colorant is a dye, preferably Orient Oil black BS. The Orient Oil black is present in a concentration of about 2.0 percent to about 8.0 percent, and preferably about 4.0 percent of the ink.
A method for producing indicia on a substrate includes the steps of heating to liquid a non-aqueous, solid hot-melt ink formulation to a temperature of not more than 130xc2x0 C., ejecting the liquid from a printing device and allowing the liquid to solidify. Preferably, the heating is carried out to maintain the ink temperature not more than 120xc2x0 C.
These and other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the appended claims.